The present invention relates to a double converter tuner for receiving a television signal such as a CATV signal having a wide bandwidth.
TV tuners using a double converter system have been recently used as wide-band tuners.
In a conventional double converter tuner, an unnecessary channel band of an input TV signal (f.sub.RF) is attenuated through a band-pass filter, and the filtered signal is frequency-converted to a first intermediate frequency or IF (f.sub.I1) signal having a frequency which is several times the input TV signal. An unnecessary frequency component of the first IF signal is removed by an IF filter. The filtered first IF signal is converted to a second IF signal having a standard intermediate frequency f.sub.I2.
In the double converter system having the configuration described above, a spurious component is easily generated by the input signal and local oscillation signals from the two IF converters when the first intermediate frequency is lowered. In order to remove the spurious component, the performance of the IF filter must be improved. However, improvement in filter performance results in a large size of the device. In addition, the band characteristics of the IF filter greatly influence interference characteristics. Therefore, an attenuation band must have a high suppression coefficient, and a loss in the passband becomes large. As a result, a noise factor (NF) is increased. Furthermore, frequency interference between a local oscillator of a first IF converter and a local oscillator of a second IF converter tends to occur, so that complete and comprehensive shielding must be performed. In addition to this disadvantage, the filter or the like in the input section has a wide band, and a specific frequency band is not selected. Therefore, the input section cannot remove interference waves.
In the conventional system of this type, an interference removal capacity, suppression of a spurious component, cross modulation characteristics, intermodulation characteristics, and the like must be improved.